The present invention relates to alignment systems and more specifically to an automated method of handling and processing printed circuit board panels, printing plates, or other sensitized sheets.
It is known today that printed circuit boards may be composed of several PCB panels, each panel having two sides, one or more of which is provided with a layer forming an electrical circuit. When there is only one panel having only two layers, the board is commonly called a double-sided board, and when there are more than two layers, the board is commonly called a multi-layer board. A common way of manufacturing a multi-layer board is by fixing several panels together, each panel having a single printed circuit on one side, or a circuit on each side. xe2x80x9cOuterxe2x80x9d panels are those that face the outside of a multi-layer PCB, and xe2x80x9cinner panelsxe2x80x9d are the interior panels. Typically, the inner panels have a circuit on both sides, while the outer panels have a circuit only on one, the outer side.
Each inner panel resembles a thin double-sided PCB in that the panel is comprised of an insulating substrate, which is clad on both sides with metallic foil, typically copper foil. A printed circuit is formed on any circuit side of an inner panel by that side""s metal cladding having a light-sensitive layer laid on top of the metal. The light-sensitive layer is exposed to light (typically ultra-violet (UV) radiation) at selected locations, then processed by a photographic process that removes the layer at selected locations. An etching process is then applied to remove those parts of the layer of metallic foil not necessary for forming the actual circuit. Once all the double-sided inner panels are produced, they are fused (pressed) together by placing an insulating binding material, typically a partially cured epoxy-resin material called prepreg, between the panels. Unexposed outer foils are placed on the outside of the double-sided inner panels, again with prepreg in between. All the layers are now laminated by applying heat and pressure that causes the prepreg to flow and bond to the surfaces of the inner panels and the outer foils. Holes are now drilled on the laminated multi-layer board, including holes for mounting electrical components inserted into the board (xe2x80x9cmounting holesxe2x80x9d), and holes for making contacts from one layer to one or more other layers (feed-throughs, also called vias or conductive vias). The holes typically are plated through. Each side of the multi-layer panel now is sensitized, then exposed and processed to form the two outer printed circuits in exactly the same manner as forming circuits on the inner panels.
Since a multi-layer panel is exposed in the same way as an inner PCB panel, the words xe2x80x9cPCB panelxe2x80x9d or simply panel will mean either a complete PCB board, an inner PCB panel, or a post-lamination multi-layer panel.
One difficulty in producing multi-layered printed circuit boards is the strict requirement for accuracy in positioning the different PCB panels together to ensure that the different circuits are positioned very accurately relative to each other. In particular, mounting holes and vias need to be very accurately placed on each layer""s circuits. For a particular tolerance for the placement of a circuit, it is clear that any deviations in the specified location of each layer""s circuit may be additive, so that at any one location, there could be large deviations. For the case of double-sided panels, including a multi-layer panel after lamination, it is even more difficult to position the circuits accurately enough relative to each other.
New technology for making PCB panels, like sequential build up (SBU) or direct ablation of the copper can be used with direct imaging technology. See below for a discussion of direct imaging. For technologies such as SBU, where each new layer is directly added to the previous stack of layers as an additive process, the relationship between the imaging process and the registration process becomes very critical so that the registration imaging processes need to interact more closely. This relationship between the imaging and registration processes becomes increasingly critical as the geometrical accuracy and PCB layout density is increased. Further, to maintain the registration accuracy through the imaging process, the registration apparatus must be accurately mated to the imaging apparatus as the need for accuracy increases.
A common method for producing printed circuit boards is to first produce artwork, which is an accurately scaled configuration used to produce a master pattern of a printed circuit. Artwork is generally prepared at an enlarged scale using various width tapes and special shapes to represent conductors. The items of artwork, once reduced, for example, by a camera onto film to the correct final size, are referred to as phototools and are used as masks for exposing the sensitized layers. Because the photographic reduction is never 100 percent accurate, more accurate phototools are produced nowadays using photoplotters rather than photographic reduction.
However produced, physical phototools are susceptible to damage and distortion. In addition, whenever any amendments need to be made to any circuit, new phototools need to be produced. Furthermore phototools, sometimes in the form of photographic negatives, are difficult to store. They also may not be stable; their characteristics might change with temperature and humidity changes and can suffer degraded quality over time. Some changes include image degradation, localized or global shrinkage or expansion.
There thus are advantages to directly imaging the required circuit patterns onto PCB panels, for example PCB panels that include a light-sensitive layer on one or both sides. The same advantage also is applicable to directly imaging printing plates that include a UV, visible light, or thermally-sensitive layer. Often such sensitive sheets as used for PCBs or thermal printing plates are rigid, so that the scanning apparatus for exposing such sheets for direct imaging (e.g., directly exposing printing plates or directly exposing PCB panels) is of the flat-bed type in which the sheet is disposed on a horizontal table for exposure by the light energy (e.g., UV light or infrared) produced by the scanner. Such scanning apparatuses are typically quite bulky because of the horizontal table. Also, such direct imaging systems expose one side at a time, and there are problems accurately aligning the two sides for double-sided exposure.
Direct imaging addresses some of the production issues such as the difficulties associated with photoplotters, phototools, and the image transfer process. Direct imaging, however, does not ensure proper alignment of the PCB panel to be processed, especially with outer layers where the image must match the drilled holes pattern. In a typical direct imaging process, a first PCB panel is imaged at a nominal 100% scale. Then the imaged first PCB panel is drilled. The imaged and drilled first PCB panel is then registered for a subsequent layer to be added. The entire image of the subsequent layer is scaled as determined in the registration of the imaged and drilled first PCB panel.
Further, direct imaging, alone, does not address the handling of the PCB panels. Modern PCB panels can be large scale such as up to 24 inches in width and up to 36 inches in length (609.6 mmxc3x97914.4 mm) or even larger PCB panels are know to be used.
The manufacturing difficulties of precise alignment and handling described above are further amplified as the overall physical size of the PCB panel increases and/or as PCB circuit density increases (e.g., line widths get smaller and closer together). In many specialized applications the PCB panel can be a large scale PCB panelxe2x80x94as large as 24 inches in width and 36 inches in length (609.6 mmxc3x97914.4 mm) or even larger. Large-scale sizes are more difficult to handle and accurately align for processing than more typical, smaller PCB panels. The result is a very slow, complicated and expensive production process that typically results in inconsistent product quality.
Thus there is a need for an automated method for precisely handling and aligning a pattern, for example, a drilled holes pattern, and for direct imaging both sides of a large scale PCB panel to produce a consistently high quality product at a low cost and high rate of production. Further, such a process should include the capability of handling large as well as small size PCB panels. To handle both large and small PCB panels requires that registration targets such special target shapes or drilled holes patterns are located at a wide variety of locations. Further, the wide variety of locations of registration targets requires fast, accurate, flexible registration process and apparatus that the typical fixed camera and encoder/track camera apparatus either cannot provide or would be very expensive to provide. Also, mixing to-be-imaged panels of varying sizes and thicknesses should happen dynamically and automatically without operator input or introducing unnecessary delays, and should provide the operator with total production flexibility.
Another problem with traditional PCB panels is that the misalignment may be localized. Because traditional PCB panels are assembled using phototools as described above, one portion of a PCB panel may be slightly out of scale or distorted, resulting in a localized misalignment that is limited to only the distorted portion of the PCB panel.
One method to resolve the above-described panel alignment issues uses a complex, expensive visual registration system. One such prior-art visual registration system is illustrated in FIG. 1 and includes a first frame 130 to mount a first PCB panel 100 in a known position and a second frame 140 to mount a second (or subsequent) PCB 120 panel therein. The visual registration system also includes an alignment mechanism 150 for aligning the frame 140 in the X-axis and Y-axis as well as any rotation 152 error (xe2x80x9cdelta thetaxe2x80x9d rotation error). The visual registration system also includes several cameras 160, 162, 164. Each of the cameras 160, 162, 164 is mounted on camera tracks 170, 171, 172, 174, 175 so that each of the cameras 160, 162, 164 can be moved to visually capture (xe2x80x9cseexe2x80x9d) an assigned registration target 180, 182, 184 respectively, on the second PCB panel 120. The registration targets 180, 182, 184 are illustrated as separate elements such as cross marks or drilled holes. The registration targets 180, 182, 184 can also be elements of the circuit on the second PCB panel 120. The camera tracks 170, 171, 172, 174, 175 must be very stable and firm so that the camera locations relative to the second frame 140 are known to a very small tolerance.
For one version, very precise and thus expensive encoders and possibly translation stages are included in the camera tracks 170, 171, 172, 174, 175. The encoders accurately determine the locations of cameras 160, 162, 164 along camera tracks 170, 171, 172, 174, 175. Each of the cameras 160, 162, 164 is moved along the tracks 170, 171, 172, 174, 175 to focus and center on the assigned registration target 180, 182, 184 respectively. Because the locations of the cameras 160, 162, 164 are known, and the cameras 160, 162, 164 are focused and centered on the respectively assigned registration targets 180, 182, 184, then the location of the registration targets 180, 182, 184 relative to the second frame 140 can be determined.
For example, determining the location of the three registration targets 180, 182, 184 in two dimensions (X-axis and Y-axis), identifies the location of the second PCB panel 120 relative to the second frame 140. If the location of the registration targets 180, 182, 184 relative to the circuit on the second PCB panel 120 are known, then the location of the circuit on the second PCB panel 120 is known. Once the location of the circuit on the second PCB 120 is known, then the second frame 140 can be adjusted with the alignment mechanism 150 until the registration targets 180, 182, 184 on the second PCB panel 120 are aligned with the corresponding registration targets 190, 192, 194 on the first PCB panel 100. Then the two PCB panels 100, 120 can be brought together for bonding and subsequent drilling, electrical connecting or other processing. The visual registration system described in FIG. 1 can also be used to accurately align a single PCB panel, such as the second PCB panel 120, for a sequential build-up (SBU) process.
The prior-art visual registration system described in FIG. 1 is complicated, requiring multiple moving cameras. The system is also expensive, requiring highly precise tracks and encoders. The prior-art visual registration system is also unable to compensate for a localized distortion of a previously assembled PCB panel. Further, the prior-art visual registration system is also unable to quickly locate a large number (such as 25 or more) of registration targets.
A method of registering panel is disclosed. First, the panel is held in a frame. The frame includes a transparent plate that covers the panel. The transparent plate has a pattern on at least one surface. Next, an image of each one of a number of registration targets on the surface of the panel is received. The image is received through the transparent panel. The actual location of each one of the registration targets relative to the pattern is then determined.
Locating the registration targets by comparison to the grid elements provides significant advantages over the prior-art methods that rely on moving cameras. Relying on the comparison of the pattern and the registration targets eliminates moving parts and provides simpler, less expensive registration engine components without reducing the accuracy of the results. In addition, interposing the pattern over the registration targets allows compensation for localized deformations.
A system for registering a panel is also disclosed. The system includes a frame for detachably supporting the panel. A transparent plate is mounted on the frame. The transparent plate covers the panel and includes a pattern on at least one surface. A vision system is also mounted on the frame such that the vision system can see the panel through the transparent plate.